There are many types of directional filters, for example, as disclosed in U.S. Pat. Nos. 2,922,123 entitled "Directional Filters for Strip-line Transmission Systems"; 3,447,102 entitled "Microwave Frequency Converting Comprising Multi-port Directional Couplers"; and 4,287,605 entitled "Directional Filter for Mixers, Converter and the Like", which are generally indicative of the state of the art. The directional filters in most, if not all, prior art systems including those mentioned above use strip-line transmission lines in close proximity to achieve the RF-to-LO frequency isolation while suppressing undesired frequencies. Unfortunately, transmission line-type directional couplers tend to require large physical areas and large coupling coefficients to minimize conversion loss which demands more LO (Local Oscillator) power. For example, a 10 dB directional coupler will increase the required LO power ten fold. The excessive power consumption and space requirements are undesirable especially in applications using MMIC (Monolithic Microwave Integrated Circuit) technology.
Hence, there is a need for an improved directional filter implemented without transmission lines and of appropriate physical size for use in MMIC technology.